1. Field of the Invention
The present invention relates to a liquid crystal display device which includes: a first and a second substrates; columnar spacers for keeping the gap therebetween; and a liquid crystal material filled in the gap.
2. Description of the Related Art
In the liquid crystal display device, spacers are inserted between two substrates in order to uniformanize the thickness of a liquid crystal layer between the two substrates. The columnar spacer is a spacer of a columnar shape provided to either one of the two substrates, and the tip comes to abut against the opposing substrate to keep the specific gap between the two substrates. When the columnar area density determined according to a product of the number of the columnar spacers per unit area and the area of the single columnar spacer is decreased, a following issue occurs. Since the anti-load property between the two substrates becomes insufficient, a plastic deformation occurs in the columnar spacers of that part particularly when a strong force is applied locally. As a result, the shape of the columnar spacers such as the height thereof becomes changed, so that the local gap unevenness is generated and may look as a display unevenness in some cases.
In the meantime, elastic deformation of a prescribed amount is applied in advance to the columnar spacers. The reason for this is to prevent the columnar spacers from becoming detached from the opposing substrate when the volume of the liquid crystal is expanded in accordance with an increase in the temperature. When the columnar area density is increased in such state, a frictional force between the columnar spacers and the substrate generated due to the elastic deformation is increased. Thus, even when there is a shifting stress (a force to shift the two substrates in parallel to each other) is generated between the two substrates, there is no shift generated therebetween due to the friction force. As a result, distortion is accumulated within the substrate (e.g., glass substrate), and the black display becomes nonuniform due to the elastic optical effect caused by the distortion.
In order to ease the trade-off mentioned above, there is proposed a technique which disposes pixels each having a step film on the substrate side opposing to the columnar spacers and pixels not having the step film on the substrate side opposing to the columnar spacers on a display panel (see Japanese Unexamined Patent Publication 2005-338770 (Patent Document 1), for example). For the pixel having the step film, elastic deformation of a prescribed amount is applied in advance to the columnar spacer (i.e., the tip of the columnar spacer is brought to abut relatively hard against the step film forming part), so that the elastic deformation can be maintained even when the volume of the liquid crystal is expanded due to an increase in the temperature. In the meantime, for the pixel having no step film, almost no elastic deformation is applied to the columnar spacer (i.e., a gap is provided between the tip of the columnar spacer and the opposing substrate, or, the tip of the columnar spacer is brought to abut relatively lightly against the opposing substrate), so that the tip of the columnar spacer comes to abut relatively hard against the opposing substrate only when there is a strong force applied between both substrates to function to support the columnar spacer of the pixel that has the step film. Such columnar spacer of the pixel having no step film along with the peripheral structure thereof is called an auxiliary column structure.
Hereinafter, the technique depicted in Patent Document 1 will be described as a “related technique”. FIGS. 10A and 10B show a pixel having a step film according to the related technique, and FIGS. 11A and 11B show a pixel having no step film according to the related technique.
According to the related technique, on a TFT (Thin Film Transistor) substrate 1201, a pixel 1001 having a step film 1100 as a step film forming part is formed on the TFT substrate 1201 side which opposes to a columnar spacer 1304 as shown in FIGS. 10A and 10B and a pixel 1002 having no step film is formed on the TFT substrate 1201 side opposing to a columnar spacer 1305 as shown in FIGS. 11A and 11B. The columnar spacers 1304 and 1305 are provided on a counter substrate 1202 to keep the gap between the TFT substrate 1201 and the counter substrate 1202. A liquid crystal 1500 is sealed between the TFT substrate 1201 and the counter substrate 1202 by a well-known method. Both of the columnar spacers 1304 and 1305 are fixed (formed) on the inner face of the counter substrate 1202, i.e., on a surface opposing to the TFT substrate 1201. Further, as shown in FIGS. 10A and 10B, the step film 1100 as a step film forming part is formed at a position corresponding to the columnar spacer 1304 on the inner face of the TFT substrate 1201. The tip of the columnar spacer 1304 abuts against the corresponding step film 1100 at all times. As shown in FIGS. 11A and 11B, there is a gap corresponding to the film thickness of the step film 1100 between the tip of the columnar spacer 1305 and a corresponding part in the TFT substrate 1201 in a normal state (i.e., in a state where an external force is not applied from the outside).
FIG. 10A is an enlarged plan view of the pixel 1001 in which the columnar spacer 1304 is formed. In the pixel 1001, provided are a scan line 1011, a common wiring 1012, an inorganic insulating film 1021, a transistor (amorphous silicon: a-Si) 1032, a signal line 1040, a pixel wiring 1044, a protection film 1045, a pixel electrode 1071, a common electrode 1072a, a shield common electrode 1072b, the step film 1100, a common-electrode contact hole 1101a, a pixel-electrode contact hole 1101b, and the columnar spacer 1304.
FIG. 10B is a cross-sectional view of a part including the columnar spacer 1304 of FIG. 10A. In FIG. 10B, on the inner surface (i.e., the top face in the drawing) of the TFT substrate 1201, provided are a scan line 1011a containing Al (aluminum) as a main component, for example, a scan line 1011b containing Cr (chrome) as a main component, for example, a common wiring 1012a containing Al as a main component, for example, a common electrode 1012b containing Cr as a main component, for example, an inorganic insulating film 1021, a protection film 1051, an organic film 1061, and the step film 1100 formed at a position corresponding to the columnar spacer 1304. The step film 1100 is constituted with a first Cr layer 1041, an Al layer 1042, and a second Cr layer 1043 formed in order on a lowermost a-Si layer (step film) 1031.
In the meantime, on the inner surface of the counter substrate 1202 (i.e., the bottom face in the drawing), a black matrix 1301 having a light-shielding function, a color layer 1302, and a protection layer 1303 are formed, and the columnar spacer 1304 of a prescribed height is formed and attached on the protection layer 1303. Further, the liquid crystal layer 1500 is sandwiched or inserted (sealed) between the TFT substrate 1201 and the counter substrate 1202.
FIG. 11A and FIG. 11B are fragmentary enlarged views of the pixel 1002 corresponding to the columnar spacer 1305, and FIG. 11A is a plan view while FIG. 11B is a cross-sectional view of a part containing the columnar spacer 1305 of FIG. 11A. FIG. 11A is similar to FIG. 10A, except that the step film 1100 is not formed in FIG. 11A. Further, FIG. 11B is similar to FIG. 10B, except that the columnar spacer 1305 is formed instead of the columnar spacer 1304 of FIG. 10B and that the step film 1100 is not formed in the TFT substrate 1201 at the position corresponding to the columnar spacer 1305. Therefore, the tip of the columnar spacer 1305 is isolated from the protection film 1051.
As described above, the columnar spacer 1304 shown in FIGS. 10A and 10B is a columnar spacer which abuts against the step film 1100 formed on the opposing TFT substrate 1201 side at all times. In the meantime, the columnar spacer 1305 shown in FIGS. 11A and 11B is a columnar spacer which abuts against the TFT substrate 1201 only when the gap between the TFT substrate 1201 and the counter substrate 1202 is narrowed, since there is no step film 1100 on the opposing TFT substrate 1201 side.
In FIGS. 11A and 11B, the step film 1100 is not formed in the part corresponding to the tip of the columnar spacer 1305. Therefore, there is a gap between the columnar spacer 1305 and the TFT substrate 1201. Only when an external force squashing the panel is applied, the columnar spacer 1305 comes in contact with the TFT substrate 1201 to keep the gap between the TFT substrate 1201 and the counter substrate 1202.
As shown in FIGS. 10A and 10B, with the related technique, the step film 1100 of the columnar spacer 1304 is formed on the scan line 1011a. In that case, a semiconductor layer is interposed between two metal layers which form the wiring to form two kinds of pixels with a part having the semiconductor layer (step film) and a part not having the semiconductor layer. In the pixel having the semiconductor layer, the columnar spacer abuts against the step film forming part to support the gap between both substrates. In the meantime, in the pixel not having the semiconductor layer, the columnar spacer does not abut against the opposing substrate but comes to become floated, and the columnar spacer abuts against the opposing substrate to function as an auxiliary column for easing the load only when a strong load is applied locally.
As described above, the auxiliary column in which one or more kinds of inorganic layers (Cr layer, Al layer, a-Si, etc.) are combined as the step film forming part is proposed in the related technique. In this related technique, the step film of the columnar spacer is formed in a region on the scan line where a storage capacitor is not formed. Further, it is presupposed that the step film is formed not for all the pixels but only for a part of the pixels.
With a liquid crystal display device, it is necessary to form wirings other than the wiring for forming the storage capacitor to be thin as much as possible in order to acquire a high numerical aperture. For example, when forming the storage capacitor on a common wiring for supplying a common potential, it is desirable in terms of the numerical aperture to form the scan line where no storage capacitor is formed to be thin as much as possible. When the step film is to be formed on the scan line under such condition, the step film becomes extremely smaller compared to the columnar spacer. Thus, it is not possible to support the columnar spacer stably.
Therefore, in order to satisfy the demand for acquiring the higher numerical aperture, the step film needs to be formed on the common wiring that is formed relatively thick for forming the storage capacitor. However, the storage capacitor formed on the common wiring occupies the most of the common wiring region, so that the step film needs to be formed in the storage capacitor part. In the meantime, when the step film is formed in the region other than the storage capacitor part, a special region is required. This results in deteriorating the numerical aperture.
Similarly, it is also necessary to form the common wiring to be thin as much as possible even when the storage capacitor is formed on the scan line. Thus, the step film needs to be formed in the storage capacitor part on the scan line.
As described, for pursuing the higher numerical aperture, it is desirable to form the step film in the storage capacitor part. When the step film is to be formed only for prescribed pixels as in the case of the related technique, the step film needs to be formed with a passivation insulating film formed on the second metal layer or with another inorganic film layer in a case where the step film is to be formed with an electrode that constitutes the storage capacitor. In the former case, it is necessary to eliminate the passivation film other than the step film, so that it is not possible to secure a sufficient area in the region where the storage capacitor is formed. Further, the electrode is exposed in a wide area, so that there is an issue generated in terms of the reliability. In the latter case, there is an increase in the number of steps.
It is therefore an exemplary object of the present invention to provide an electrically stable step film structure which can be formed in the region of the storage capacitor without increasing the number of steps.